How many minutes / hours a day do you spend playing? How many days / weeks/months it normally takes between character creation and the end of the game?
I usually create a character and do not close the game until the character reaches about cL20. Then, when I have a desire, I play several times a day for 20-40 minutes games. And so on until the death of my character The most long-lived character I played for two weeks, and then won.

Also tell us about your favorite part of the game.
I like the beginning of the game, and the late stage. Because at the beginning of the game I was very pleased with the good finds and killing some monsters. And at the end of the game I enjoy the power of my character and I feel invincible. The middle of the game, I do not like. Usually I get bored in the middle. I forget about caution and begin to behave downright dangerous. Therefore most of my characters dies.

When I'm playing Vanilla or 100 level variants, I play in 2-3 hour segments, usually drinking something strong, *or*, I'm playing at work, half distracted on a tiny window in the corner of my screen. My enjoyment of the game is directly proportional to how much damage I can dish out, so, I'm not really even having any fun until middle to end game.

Also tell us about your favorite part of the game.
I like the beginning of the game, and the late stage. Because at the beginning of the game I was very pleased with the good finds and killing some monsters. And at the end of the game I enjoy the power of my character and I feel invincible. The middle of the game, I do not like. Usually I get bored in the middle. I forget about caution and begin to behave downright dangerous. Therefore most of my characters dies.

I suspect that might be a consequence of playing too many mages :P
Those have a long "tedium-phase" in midgame, other classes less so.

For PosChengband, I generally do this:
A) When creating Possessors, I constantly create characters until I get a favorable combination of my TOP 3 guaranteed opportunities:

i.e., at least TWO of the following in most cases --
1) Locke, the Superman
2) Combat-Echizen "Because it's time"
3) Wahha-Man the Golden

Hence, the ongoing tradition of my PosChengband Hugo$oul series. This character creation process of mine may take up to 30 minutes or longer in some cases.

B) When playing Possessors, I play for 1-2 hours at a time. Sometimes, I will stop much sooner if I reach a key point, like possessing a Saber-tooth tiger.

C) When creating non-Possessor characters, I play them for about 1-2 hours at a time, sometimes stopping after certain quests or a certain hour.

For Sil, I play for up to 1 to 1 1/2 hours and then stop, usually at 500ft-700ft.

My favorite parts:
PosChengband
--early-mid game: because I can get lucky finds from the various town quests
--late game: when I get really advanced (e.g., clvl 50 Possessor), the game gets real when I possess monsters like Locke, the Superman

Sil
--mid-late game: this part of the game feels easy for some reason, partly because there's a chance I can get my favorite Greatsword in the 600ft-700ft range.

The game brings more fun with a priest or a rogue? When my current mage will die, i will create one of these classes.

Mages are infamous for hitting a massive slump in the midgame, where their ability to kill things takes a nosedive. For the other classes:

Warriors: start out immensely strong; hit a minor slump in the midgame when they lack detection, but that stops being a problem once they get telepathy and/or a Rod of Detection.
Paladins: are basically strong throughout, and nigh-unkillable in the endgame.
Priests: Weak start, but once they get Orb of Draining they're set for a long time. Eventually they need to find another way to kill things, but that's a late-game problem.
Rogues: are basically strong throughout.
Rangers: honestly I haven't played one of these in awhile, but I get the impression they have a somewhat weak start, and are heavily reliant on finding a good bow, but once they do they can kill anything with relative ease.